TEXTURAL DAMAGE OF CATION-EXCHANGED LTA ZEOLITES STUDIED BY GAS-ADSORPTION

Textural changes induced on zeolite LTA by cation exchange were studied by gas adsorption. Adsorption-desorption isotherms of N-2 and O-2 at 77 K were determined on NaA samples exchanged by various amounts of Li+, CO2+, CU2+, and Zn2+. The t-plot technique was used, taking NaA saturated by water as the nonporous reference absorbent. t-plots are not linear, except when the crystalline structure of the sample is entirely destroyed. Nevertheless, valuable indications on the microporous volume and crystallite outer surface area are obtained, showing crystal damage induced by divalent transition cations (Cu2+>Ni2+>CO(2+)similar to Zn2+). Adsorption of N-2 in some micropores of NaA and other samples exchanged below the percolation threshold was also pointed out. The cause of the nonlinearity of the t-plots is discussed. Influence of a microporous skin is suggested. Moreover, a simple graphical method, based on the slope at the inflection point of the isotherm, is presented. It qualitatively shows whether there is simultaneously adsorption in micropores and multilayer adsorption.